To improve design quality of a vehicle, a ceramic shielding layer often is formed on a peripheral part of a vehicle window glass. The ceramic shielding layer serves also to prevent ultraviolet-induced degradation of resin materials such as a foam and an adhesive for joining the window glass to the vehicle body. The ceramic shielding layer is formed by applying and sintering a ceramic paste on the window glass.
In some cases, vehicle window glasses are required to have the function of shielding against ultraviolet light, for example, to prevent sunburn. Also, the function of shielding against infrared light may be desired, for example, to reduce air conditioning cooling load or sense of heat. Other functions which may be required of window glasses include water repelling function and anti-fogging function. To meet the demands for these functions, window glasses having various functional thin films formed thereon are produced. Functional thin films may be formed by a vapor phase deposition process; however, a liquid phase deposition process in which a coating liquid is fed to form a film is more suitable for mass production.
In general, a metal compound reactive with hydroxyl groups (OH groups) present on the surface of a glass sheet is added to the coating liquid. A typical example of the metal compound is a silicon compound typified by a silicon alkoxide such as tetraethoxysilane (TEOS) or a hydrolysate thereof. The metal compound reacts with hydroxyl groups on the surface of a glass sheet to fix a functional thin film on the surface of the glass sheet. Strictly speaking, silicon (Si) is not a metal atom. In the present description, however, the term “metal compound” is intended, as is conventional, to encompass silicon compounds.
The surface of the ceramic shielding layer has a smaller number of hydroxyl groups per unit area than the surface of the glass sheet. For this reason, when a functional thin film is formed to cover not only a transparent region in which the surface of the glass sheet is exposed but also the surface of the ceramic shielding layer, the functional thin film may peel off from the surface of the ceramic shielding layer. Unlike the transparent region, the shield region on which the ceramic shielding layer is formed is shielded by itself from light and view. Thus, loss or reduction of a function due to the peeling-off of the functional thin film poses no problem to the shield region. However, partial peeling-off of the functional thin film causes an aesthetic defect in the window glass.
In view of the above circumstances, when a ceramic shielding layer is present on a surface of a glass sheet on which a functional thin film is to be formed, a coating liquid is applied to the surface after masking of the ceramic shielding layer with a masking tape. Patent Literature 1 points out the problem of the occurrence of optical transmission distortion attributed to a local increase in thickness of the functional thin film caused by a coating liquid pool formed due to the attachment of a masking tape. To prevent this optical transmission distortion, Patent Literature 1 proposes adjusting the relationship between the total thickness of the masking tape and the ceramic shielding layer and the thickness of the film to be formed.